Storage of volatile liquids



Nov. 29, 1960 w. A. GOLDTRAP 2,961,840

' STORAGE 0F VOLATILE LIQUIDS Filed Aug. 12, 1957 RATION T REFRIGE UNIFROM REFRIGERATION UNIT INVENTOR. W. A.GOLDT RA P BY MM g A T7'ORNEVSUnited States Patent Q 2,961,840 STORAGE F VOLATILE' LIQUIDS l Walter A.Goldtrap, Bartlesville, 0kla., assignor to Phillips Petroleum Company, acorporation of Delaware Filed Aug. 12,1957, Ser. N0. 677,562

7 Claims. (Cl. 62--45) This invention relates to a method of storingvolatile liquids. In another aspect it relates to a method of preparinga reservoir for the storage of volatile liquids. In still another aspectthis invention relates to a surface earthen storage reservoir madesuitable for the storage of volatile liquids therein.

Volatile liquids which are produced in large quantities present a majorstorage problem. Products such as propane, butane and liquid ammoniarequire enormous storage facilities dur'ng seasons of slack use. In aneffort to avoid the expense of metal pressure containers, products ofthis type have been stored in underground formations provided by miningor dissolving a cavity in a soluble strata. While underground storage ofthis type has proved quite satisfactory in many instances, a suitableformation is not always readily available at the most convenient storagesite. of products which are less volatile, for example fuel oil, massivestorage facilities are provided in open pits or reservoirs, such asabandoned quarr'es, and equipped with a floatroof. This type of storage,however, has not been practicable for volatile products which develophigh vapor pressures at ambient temperatures.

According to my invention a method of storing volatile products insurface reservoirs is provided. Volatile liquids are retained in opentopped reservoirs by sealing the top opening of said reservoir with anice cap, and after introducing the volatile liquid into the reservoir,maintaining the temperature of the liquid sufficiently low that itsvapor pressure does not exceed that pressure which the ice capcan'withstand. Such a storage facility can be provided at a greatsavings in expense over metal pressure vessels and can be maintained forlong periods of time at nominal expense. A method by which I prefer toprovide such a storage facility is to use an open pit in the surface ofthe earth acquired either by excavatin or by using an already existingpit, such as an abandoned quarry, and to span the top opening of thispit with a heat exhcange conduit. After the refrigeration equipment isin place, the pit is filled with water and an ice cap is formed byfreezing a layer of ice on the surface of the water, thus sealing theopening of the pit. The ice cap thus formed is supported by the heatexchange conduits and auxiliary structure and the water is then removedfrom the pit, leaving an enclosed cavity to receive the volatile productintended for storage therein.

In the storage of other types The reservoir is then filled with volat'leproduct, such Still another object of my invention is to provide such asurface, earthen storage system for the storage of volatile liquids. Itis another object of my invention to provide a method of employingvapors of a volatile stored liquid in a refrigeration cycle to seal thestorage reservoir. A still further object is to provide a method offorming a reservoir having an ice cap for a roof. Other objects,advantages and features of my invention will be apparent to thoseskilled in the art from the following description, appendant claims anddrawings in which:

Figure 1 is a view in cross section of an earthen pit employing an icecap for a cover according to my invention;

Figure 2 is a view in cross section of a preferred type of earthen pit;and

Figure 3 is a sectional view of Figure 2 along line 3-3.

While my invention is especially important in storing volatile liquids,such as liquefied ammonia and liquefied petroleum gases, which mustnormally be stored under pressure, it can also be applied to the storageof any fluid having a freezing point below that of water. Liquids forwhich the storage system of my invention is particularly well adaptedinclude liquefied butane and isobutane.

An advantage of the system of my invention is that it can be constructedin almost any locality without regard to the underlying formations. Openpits can be used or ponds can be drained. Abandoned quarries are alsosuitable. If possible, it is desirable to locate the reservoir in animpervious surface format on, such as shale or clay. If, however,-such aformation is not available, a relatively cheap lining can be provided,such as polyethylene film, a thin wall of concrete, or rubber sheet,depending upon the type of product to be stored in the reservoir.Because of the inert character of polyethylene and its resistance tovarious solvents, I prefer to employ polyethylene film as a liner insituations where such protection is required.

Another solution to the problem of providing fluidtight walls for thereservoir is to form the pit in a permeable water bearing formationbelow the water table so that water tends to seep into the pit. In suchan applicaton, as the product is cooled to lower its vapor pressure thewater in the walls of the pit becomes frozen and thereby provides animpervious barrier to contain the product. Such an approach to theproblem of providing an impervious wall in an underground storagefacility is disclosed and claimed in Patent No. 2,796,739, issued June25, 1957, to L. P. Meade and G. F. Downs, Jr.

The refrigeration equipment which is employed for my invention is of atype well known in the art, having as basic components compressors,condensers, a power source, and pumps for moving the fluid through thesystem. Refrigeration conduit is supported across the opening of thereservoir so that refrigerant can be circulated therethrough to freezethe surrounding water. The preferred method of forming the ice caparound the refrigeration conduit is to fill the pit with water so thatthe coils are covered by about half the thickness of the desired icecap. A suitable heat exchange medium is circulated through the coilsuntil the water surrounding the conduits spanning the pit becomesfrozen. The thickness of the ice cap can vary considerably, for example,from an inch or less to several feet. The exact thickness of the ice capis not critical; however, it should be suffic ently thick that itscontinuity can be maintained. For the storage of ammonia, it isdesirable to increase the thickness of the ice cap in order to gainadditional strength in the roof and thereby permit higher vaporpressures in thereservoir. For butane storage an ice cap of about 6inches to 2 feet thick is satisfactory.

After the ice cap has been formed the Water is removed from thereservoir and a layer of insulating material is placed over the ice capto protect it from damage from the weather and to aid in keeping thesurface of the ice from melting. Any suitable type of insulatingmaterial, such as sawdust, cork, rock wool, or the like can be used. Itis also desirable to provide a continuous covering over the surface ofthe insulation to protect the ice cap from rain. A suitable protectivecovering is a layer of sawdust several inches thick over the surface ofthe ice and a film of polyethylene covering the sawdust layer. Aluminumfoil can also be used to advantage to reflect sunlight. The water-tightcovering thus formed over the ice cap should be shaped to drainproperly.

The structural strength of the roof thus provided lies primarily in theheat exchange conduit and its accompanying structural members. Since theheat exchange coils lie within the ice cap they serve to reinforce itand give the roof considerable strength. The ice itself then serves toprovide a continuous covering sealing the top of the reservoir,preventing the escape of volatile liquid to the atmosphere andprotecting the stored product from contamination. To further describethe advantages of my invention, reference is now made to theaccompanying drawings.

In Figure 1, an open pit or reservoir 11 is shown containing a storedvolatile product. The upper edge 12 of reservoir 11 is shaped to providesupport for the heat exchange conduits. A concrete slab 13 is formed onthe upper edge to aid in supporting the ice cap and prevent caving ofthe earthen ledge. Refrigeration unit 14 contains the necessarycompressors, condensers, pumps and motors and provides the coolingcapacity required to circulate a refrigerant through conduit 16 shownspanning the top opening of pit 11. Heat exchange conduit 16 issupported from the floor of the reservoir by support members 17 and 13.Temperature sensing elements 19, 20, 21 and 22 are positioned to lie inthe ice cap and thereby enable automatic control of the refrigerationunit to maintain the ice cap at freezing temperatures. In addition,temperature sensitive elements 23, 24, 26, 27, 2-8, 29 and 30 are spacedin the walls of the reservoir and enable operation of the refrigerationunit to maintain the temperature of said Walls below 32 F. In thismanner an impervious ice barrier is provided within the walls and floorof the storage reservoir. Such temperature sensitive elements and acontrol system of this type is shown and explained in detail in theabove-mentioned Patent No. 2,796,739, issued to Meade and Downs.

Temperature sensitive elements in the walls of the reservoir and in theice cap communicate to refrigeration unit 14 through leads 31 and 32. Itis desirable to keep the temperature of the ice cap and the walls of thesurrounding water bearing formation below about 30 to 31 F. Anyelevation in temperature starts the refrigeration unit and thus keepsthe ice from melting.

To form the ice cap, water is pumped into the reservoir by pump 33through conduit 34. After the ice cap has been formed the water isremoved by reversing pump 33 or by displacement with the hydrocarbon tobe stored in the pit. During the formation of the ice cap refrigerant iscirculated through cooling conduit 16 and returned through line 36,valve 37 and return line 38 to the refrigeration unit where it iscompressed and condensed in a refrigeration cycle and recirculatedthrough conduit 16. After water has been removed from the reservoir andvolatile product stored therein, it is desirable to employ the vapors ofthe volatile product as the refrigerant. In this case valve '37 isclosed and valve 39 in line 40 opened so that refrigerant from coil 16is passed into the vapor space of the reservoir. Valve 41 in line 42 isopened and vapors from the reservoir pass through line 38 to therefrigeration unit. In this way the reservoir with its stored productbecomes .a part of the refrigeration system and the product is cooled byautorefrigeration. Ice cap 43 is protected with an insulating layer 44which, as has been explained previously, can be any insulating materialor combination of materials, such as sawdust covered with a film ofpolyethylene. Product is introduced into the reservoir through line 46and removed through line 47 as needed. The reservoir in use contains abody of volatile fluid 48 with a vapor space 49 between the liquid andthe ice cap. Any water leaking into the reservoir becomes frozen andforms a lower layer 50 which provides an impervious bottom in thereservoir.

To protect the ice cap from possible rupture due to high vapor pressureof the stored product, presure sensitive element 51 communicates withthe vapor space 49 through tube 52. An increase in pressure in thereservoir beyond a predetermined limit fixed to protect the ice capproduces an output which is transmitted to refrigeration unit 14 throughleads 31 and 32. Thus the refrigeration unit is operated to maintain thepressure of the reservoir at a safe value by cooling the stored product.For example, when storing butane, the pressure can be maintained atatmospheric which in turn will keep the butane slightly below 32 F.

Other means can be provided to refrigerate the system althoughautorefrigeration is the most economical. Additional cooling coils canbe provided in the reservoir itself as Well as in the ice cap and a heatexchange medium, such as brine, circulated through the coils to removeheat from the system. In this manner the refrigeration unit can beindependent of the material being stored in the reservoir.

For economy of construction both in forming the earthen pit and inplacing the heat exchange conduit spanning the upper opening thereof, Iprefer to employ an elongated trench as shown in Figures 2 and 3. Inthese figures reference numerals correspond to those used in Figure 1showing the volatile product 48, the vapor space 49, the ice cap 43 andthe layer of insulation 44 with a layer of ice 50 in the bottom of thetrench. Water is both introduced and removed through line 34 whenforming the ice cap, and hydrocarbon is introduced through line 46 andremoved from the trench through line 47. The refrigeration cycle is thesame as that described in connection with Figure l, vapors beingwithdrawn from the vapor space of the reservoir and passed through arefrigeration cycle. Liquefied vapors are permitted to pass through theheat exchange conduit in the ice cap and evaporate, thus removing heatfrom the ice cap and keeping the temperature thereof below 32 F. Asshown in Figure 3, a preferred construction of the trench permits awider average width than the width of the ice cap sealing the upperopening. A long but relatively shallow trench is easily and cheaplyconstructed. By narrowing the upper opening, the amount of auxiliarysupport required for the heat exchange conduit and ice cap is reduced.

To give added strength to the ice cap and increase the efficiency of theinsulating layer, a layer of sawdust is floated on the water filling thereservoir While the ice cap is being formed. By agitating the surface ofthe water, it is possible to incorporate some of the sawdust into theice layer. The frozen mixture of ice and sawdust is quite strong andforms an excellent insulating layer for the upper surface of the icecap.

Advantages of this invention are illustrated by the following example.The specific dimensions and conditions are presented as being typical,and this example should not be construed to limit the invention unduly.

An earthen trench 400 feet long and 15 feet deep with an average widthof 50 feet is excavated to provide a storage capacity' of approximately55,000 barrels. The Walls of the trench are excavated in a concavefashion so that the upper opening is only 40 feet in width. Heatexchange conduit is supported across the opening of the trench and thereservoir is filled with water. An ice cap 400 feet long by 40 feet wideby 6 inches thick is formed by refrigerating the surface of the waterfilling the trench.

The formation of the ice cap is completed in two days with a 150horsepower refrigeration unit. After the ice cap is formed, a 3 inchlayer of sawdust is placed over the surface thereof and the surface ofthe sawdust is protected with polyethylene film 6 mils in thickness. Thewater is then pumped from the trench and replaced with butanerefrigerated to a temperature of approximately 32 F. The pressure in thereservoir is maintained at 14.5 pounds per square inch absolute and theice cap and butane at 31 F. As the surrounding earth becomes cooled bycontact with the refrigerated butane, heat transferred from the earthprogressingly decreases. After operating for several days, substantialequilibrium is reached and the entire system is refrigerated with ahorsepower unit.

Storage system of my' invention is relatively cheap to install and canbe maintained at low expense. Since the investment is low and theequipment utilized of the nature which can be readily dismantled andmoved to a 1 new location, the storage system described is excellent fortemporary large capacity storage of volatile liquids.

As will be evident to those skilled in the art, various modifications ofthis invention can be made or followed in the light of the foregoingdisclosure and discussion without departing from the spirit or scopethereof.

I claim:

I. An earthen storage system for normally gaseous hydrocarbon liquidscomprising, in combination, an earthen reservoir having a top opening,refrigeration conduit spanning said opening, an ice cap surrounding saidconduit sealing said opening, said conduit both refrigerating andsupporting said ice cap, protective insulating material covering theupper surface of said ice cap, a refrigeration plant equipped toreceive, compress, and condense vapors from said reservoir and expandsame through said conduit in a refrigeration cycle, said conduitterminating in said reservoir, and means for sensing ice cap temperatureand reservoir pressure and operating said refrigeration plant tomaintain said temperature and pressure below predetermined limits.

2. A system according to claim 1 wherein said reservoir contains animpermeable liner.

3. A system according to claim 1 wherein said protective insulatingmaterial comprises sawdust, at least a portion of which is integral withsaid ice cap.

4. An earthen storage system for normally gaseous hydrocarbon liquidscomprising, in combination, a trench in a permeable water-bearingsurface formation, said trench having a top opening, refrigerationconduit spanning said opening along the length of said trench, a layerof ice surrounding said conduit and refrigerated and supported by saidconduit sealing said opening, a protective covering of insulatingmaterial over the upper surface of said layer of ice, a. refrigerationplant, means for Withdrawing vapor from said trench to thereby coolvolatile hydrocarbon stored therein by autorefrigeration, means forpassing said vapor through said refrigeration plant and returning sameto said trench through said conduit to thereby cool said ice layer,temperature sensitive elements positioned in said ice layer and in saidformation adjacent said trench, a pressure sensitive element in saidtrench just below saidice layer, and an electrical circuit incombination with said sensitive elements and refrigeration plant wherebyrefrigeration can be commenced to maintain said ice layer and wateradjacent said trench in said formation frozen and the pressure in saidtrench below a predetermined value.

5. An earthen storage system for normally gaseous hydrocarbon liquidscomprising, in combination, an earthen reservoir in a permeablewater-bearing surface formation having a top opening, refrigerationconduits spanning said opening, a layer of ice surrounding said conduitand refrigerated and supported by'sa'id conduit sealing said opening, aprotective covering of insulating material over the upper surface ofsaid layer of ice, a refrigeration plant, means for withdrawing vaporfrom said reservoir to thereby cool volatile hydrocarbon stored thereinby autorefrigeration, means for passing said vapor through saidrefrigeration plant and returning same to said reservoir through saidconduit to thereby cool said ice layer, temperature sensitive elementspositioned in said ice layer and in said formation adjacent to saidreservoir, a pressure sensitive element in said reservoir just belowsaid ice layer, and an electrical circuit in combination with saidsensitive elements and refrigeration plant whereby refrigeration issupplied to maintain said ice layer and water .adjacent said reservoirand in said formation frozen and the pressure in said reservoir below apredetermined value.

6. An earthen storage system for a normally gaseous liquid comprising anopen-topped earthen reservoir; refrigeration conduit means spanning theopening of said reservoir; an ice cap surrounding said refrigerationconduit means to seal said opening, conduit means for intro ducing anormally gaseous liquid into said reservoir thus sealed; a refrigerationplant equipped to receive, compress and condense vapors; conduit meansfor withdrawing vapors of liquid from said reservoir and directing sameto said refrigeration plant; and means for expanding the returningcondensed vapors to said refrigeration conduit means to said reservoir.

7. An earthen storage system for a normally gaseous liquid comprising anopentopped earthen reservoir, refrigeration conduit means spanning theopening of said reservoir, an ice cap surrounding said refrigerationconduit means to seal said opening, protective insulating materialcovering the upper surface of said ice cap, conduit means forintroducing a normally gaseous liquid into said reservoir, means forsensing the pressure of vapors within said reservoir, means for sensingthe temperature of said ice cap, conduit means for removing vapors fromsaid reservoir, means for compressing and condensing the vaporswithdrawn from said reservoir, and conduit means for returning thecondensed vapors to said reservoir through said refrigeration conduitmeans in response to said temperature sensing means and said pressuresensing means.

References Cited in the file of this patent UNITED STATES PATENTS204,210 Gangee May 28, 1878 2,303,427 Bittner Dec. 1, 1942 2,437,909Cooper Mar. 16, 1948 2,682,752 Branson July 6, 1954 2,796,739 Meade etal June 23, 1957

